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Re: Silly question about ARM thumb


On Tue, 2005-02-22 at 14:25 +0100, Øyvind Harboe wrote:
> If the code is smaller and faster, why aren't all ARM programs compiled
> w/ARM thumb?
> 
> Tell me about the downside, I can live with the upside :-)
> 
> I'm pretty happy with code-size on the Atmel EB40a, so I'm a bit 
> averse to changing things.
> 
> This posting quotes 20% code size decrease and 50% speed increase.
> 
> http://sourceware.org/ml/ecos-discuss/2002-11/msg00036.html

In general, I'm not sure about the speed up.  The biggest shortcoming
I can think of is that THUMB mode is pretty restrictive - it has serious
limits on addressing and program size.  Also fewer available registers
(although few compilers make stellar use of all the existing ARM 
registers)  Finally, I think that on a system with 32 bit wide memory
that has no cache, THUMB would suffer some performance hits as all
instruction accesses are 16 bit only, so a lot of memory bandwidth
would just get wasted.

Just my 2c - THUMB mode is interesting, but I see it being useful
in only a small set of circumstances.

-- 
------------------------------------------------------------
Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
------------------------------------------------------------


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